U.S. patent number 3,980,405 [Application Number 05/505,778] was granted by the patent office on 1976-09-14 for cathode-ray tube picture projection apparatus.
This patent grant is currently assigned to Hitachi, Ltd.. Invention is credited to Takatoshi Ikeda, Yujiro Tatsuno.
United States Patent |
3,980,405 |
Tatsuno , et al. |
September 14, 1976 |
Cathode-ray tube picture projection apparatus
Abstract
To prevent a picture of cathode-ray tube, which picture is
projected through an optical lens system from being imaged with a
deformed shape on a flat plane because of the curved phosphor
screen of cathode-ray tube, an optical lens system is provided
having imaging characteristics such that the central portion of an
image formed thereby is convex toward the optical lens system.
Inventors: |
Tatsuno; Yujiro (Hitachi,
JA), Ikeda; Takatoshi (Hitachi, JA) |
Assignee: |
Hitachi, Ltd.
(JA)
|
Family
ID: |
26351651 |
Appl.
No.: |
05/505,778 |
Filed: |
September 13, 1974 |
Foreign Application Priority Data
|
|
|
|
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Sep 17, 1973 [JA] |
|
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48-103789 |
Feb 8, 1974 [JA] |
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49-15492 |
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Current U.S.
Class: |
355/20; 348/776;
355/52 |
Current CPC
Class: |
G02B
13/00 (20130101); G02B 13/16 (20130101); G03B
31/02 (20130101); H04N 1/401 (20130101) |
Current International
Class: |
G03B
31/00 (20060101); G03B 31/02 (20060101); G02B
13/00 (20060101); G02B 13/16 (20060101); H04N
1/401 (20060101); G03B 027/76 (); G03B 027/78 ();
G03B 027/68 (); H01J 029/89 () |
Field of
Search: |
;355/20,1,72,52
;178/7.85 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Wintercorn; Richard A.
Attorney, Agent or Firm: Craig & Antonelli
Claims
We claim:
1. An apparatus for projecting a cathode-ray tube display onto a
flat imaging plane comprising a cathode-ray tube having a curved
phosphor screen, said cathode-ray tube including means for
displaying a picture on said curved screen, wherein said means for
displaying said picture includes means for controlling the
brightness of said phosphor screen of said cathode-ray tube to be
substantially the same at the peripheral portion and at the central
portion, and optical means for projecting said picture on said
curved screen onto a flat imaging plane, said optical means having
imaging properties for imaging a flat object plane onto a curved
imaging surface with a central portion of said curved imaging
surface being convex toward said optical means, such that an image
of said picture on said curved screen is imaged on said flat
imaging plane, and wherein said means for controlling the
brightness of said phosphor screen of said cathode-ray tube
includes first circuit means for producing in accordance with a
horizontal synchronizing signal a first parabolic output voltage
which increases from the center of deflection to the peripheral
portions of deflection for a horizontal scan period, and second
circuit means for producing in accordance with a vertical
synchronizing signal a second parabolic output voltage which
increases from the center of deflection to the peripheral portions
of deflection for a vertical scan period, said parabolic output
voltages of said first and second circuit means forming control
signals to control the brightness of said phosphor screen.
2. An apparatus according to claim 1, wherein a photosensitive
recording medium is situated at said flat imaging plane.
3. An apparatus according to claim 2, wherein the photosensitive
medium is transported to said imaging plane by means of a transport
belt.
4. An apparatus according to claim 1, wherein said first circuit
means includes first integrating means for intergrating twice an
input signal corresponding to said horizontal synchronizing signal
to produce said first parabolic output signal, and wherein said
second circuit means includes second integrating means for
integrating an input signal corresponding to said vertical
synchronizing signal to produce said second parabolic output
signal.
5. An apparatus according to claim 1, wherein said optical means
includes lens means for projecting and imaging said picture on said
curved screen onto said flat imaging plane.
6. An apparatus according to claim 5, wherein said lens means
includes a lens system being constituted in sequence along the
optical axis by a first positive lens having a strong refractive
power, a first negative lens, a second positive lens having a
weaker refractive power, a third positive lens having a weaker
refractive power, a second negative lens, and a fourth positive
lens having a strong refractive power, and wherein said first
positive lens, said first negative lens and said second positive
lens are arranged to oppose said third positive lens, said second
negative lens, and said fourth positive lens with an aperture
therebetween.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an apparatus for projecting a
picture of cathode-ray tube, and more paticularly to such an
apparatus for correctly projecting a picture displayed on the
curved phosphor screen of cathode-ray tube upon a flat plane.
2. Description of the Prior Art
Recently, a technique has been utilized widely in which a picture
displayed on a cathode-ray tube (hereinafter referred to as CRT) is
projected upon a screen or a recording surface. In general, since
the phosphor screen of CRT does not take the form of a flat plane
but takes the form of a curved surface (in many applications, a
spherical surface), the picture displayed is commensurate with the
curved surface. The curved picture is projected upon a flat screen
or recording surface. In such a case, the focal point of the
optical lens system is matched with respect to the central portion
of the picture, and the peripheral portion thereof is not focussed.
If the focal point of the optical lens system is matched with
respect to the peripheral portion of the picture, the central
portion thereof is not focussed. For this reason, with this type of
projection apparatus it has been a practice to use a CRT having the
phosphor screen whose curved surface approaches as close to a flat
plane as possible. However, such an approach is often
disadvantageous from technical and economical standpoints of view.
More particularly, an enclosure of a CRT which is evacuated at high
vacuum degree is required to be strong enough for withstanding
atmospheric pressure and in addition, the phosphor screen
advantageously has a curved surface since a phosphor material is
coated on the CRT panel by precipitating a suspension. In a CRT of
7 inch screen size, for example, the curvature radius of its
phosphor screen is considered 1 m at most. Further, if the phosphor
screen takes the form of a flat plane, the diameter of a bright
spot, which luminesces under bombardment of electron beams,
increases at the peripheral portion of CRT screen. This means that
a high resolution display cannot be attained over the entire
screen. Still further, since the quantity of light is decreased at
the peripheral portion of the screen as compared to the central
portion thereof, uniform illumination cannot be obtained.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a projection upon
a flat plane apparatus capable of correctly projecting a picture
which is displayed on a curved surface plane.
Another object of the invention is to provide a projection
apparatus capable of correctly projecting a picture displayed on
the CRT phosphor screen upon a flat plane.
Yet another object of the invention is to provide a projection
apparatus capable of allowing a CRT to display on its phosphor
screen a picture of brightness suitable for the projection and of
correctly projecting the picture thus displayed upon a flat
plane.
Yet another object of the invention is to provide a projection
apparatus suitable for projecting a picture upon a photosensitive
recording medium.
Yet another object of the invention is to provide an inexpensive
projection apparatus.
The present invention is characterized in that an optical lens
system for projecting a picture displayed on a curved surface upon
an ideal (flat) imaging plane has such imaging characteristics that
the central portion of an image formed thereby is convex toward the
optical lens system.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a diagramatic representation useful to explain the
principle of a CRT picture projection apparatus.
FIG. 2 is a schematic diagram of a CRT picture recording
apparatus.
FIGS. 3a to 3c are diagramatic representations useful to explain
imaging of an optical lens system.
FIG. 4 is a schematic diagram of an optical lens system embodying
the invention.
FIG. 5 is a diagramatic representation useful to explain the
illumination intensity of a picture to be projected.
FIG. 6 is a block diagram of a brightness correction
arrangement.
FIG. 7 is a circuit diagram of FIG. 6.
FIGS. 8a and 8b are diagramatic representations of illumination
distribution.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
As shown in FIG. 1, a CRT picture is projected upon an imaging
plane 3 through an optical lens system 2. In the figure, the
optical axis of the lens system 2 is aligned in line, but otherwise
it may be refracted by means of a mirror interposed on the axis
dependent on a relation between the situation of a CRT 1 and that
of the imaging plane 3. In the case where the CRT picture is seen
by eyes, the imaging plane 3 should be a screen. In accordance with
this invention, the optical lens system 2 has such imaging
characteristics that an image formed on the imaging plane 3 is
convex at its central portion toward the optical system 2. Thus, as
to be described later, adverse affect of curved display screen 1a
of the CRT 1 upon the image formation is compensated for and the
CRT picture can correctly be projected upon the imaging plane
3.
As shown in FIG. 2, a picture recording device comprises a CRT 4
which includes a spherical phosphor surface of a predetermined
curvature and to which an external electric signal is applied so
that a picture is displayed on the spherical phosphor surface. A
picture thus displayed on the CRT 4 is subjected to an image
formation on the imaging plane 6. Numeral 7 designates a recording
paper, for example for use in xerography, which is fed out by
rollers 8 and then severed into a predetermined size by a cutter
10. The recording paper 7 is charged with electricity uniformly by
a charger 9 before it is exposed to light and thereafter carried on
a transport belt 11 so as to be situated at the imaging plane 6. A
picture of the CRT 4 is directed through an optical lens system 5
towards the recording paper 7 to expose its photosensitive surface,
and a charged latent image corresponding to the picture of the CRT
4 is obtained.
The recording paper 7 thus exposed is again transported by means of
the belt 11 and the rollers 12 to a developer 13 where the charged
latent image is converted into a visible image, and the recording
paper 7 now bearing the visible image is projected outside the
picture recording device.
With this device, in order to display, on one picture frame of the
CRT 4, either 1000 to 2000 words of English letters (characters)
and numerals or 200 to 300 words of Japanese letters containing
Chinese letters and to record them on a recording paper through an
optical lens system, the CRT 4 is need to have a notably high
resolution power.
The phosphor screen of the CRT 4, for example 7 inch high
resolution CRT, takes a form of spherical surface of about 800 mm
curvature. In this case, the amount of deviation of the phosphor
screen from the tangential plane is about 3.1 mm at a point spaced
70 mm from the center (optical axis) of picture.
On the other hand, where a picture of such CRT 4 is recorded at
magnification of about 1.5 (this magnification corresponds to an
enlargement up to A5-size sheet) and the exposure time is set to
several seconds, it is desirable to determine the aperture ratio of
the lens 5 to about 4.0.
When a lens having an aperture ratio of 4.0 and focal distance of
135 mm is used at magnification of 1.5 on the assumption that the
lens has no aberration and it can project an object on an ideal
flat plane upon an ideal flat imaging plane, it is possible to
obtain a resolution power of 5/mm on a picture of CRT 4 within the
range of a deviation from the tangential plane of 1.3 mm.
Accordingly, the CRT 4 as described above having a phosphor screen
of 800 mm radius cannot satisfy this condition.
In order to confine the deviation from an ideal imaging plane
within 1.3 mm at a point spaced from the center of picture, i.e.
from the optical axis of lens, a commensurate curvature radius of
the phosphor screen of the CRT 4 therewith is calculated to be
about 1880 mm. Therefore, in further consideration of lens
aberrations which have been assumed to be negligible, it is
difficult to manufacture the CRT 4 of high resolution.
However, by making the lens 5 for projecting a picture of the CRT 4
commensurate with the curvature of phosphor screen of the CRT 4,
i.e. by making the lens 5 to have a spherical surface of a
curvature commensurate with that of the phosphor screen for
compensating for a picture, displayed on the spherical phosphor
screen, which in turn is projected upon an ideal flat imaging
plane, a picture displayed on the CRT 4 can be made approximately
identical with an image formed on an ideal flat imaging plane.
With reference to FIGS. 3 and 4, an optical lens having such
imaging characteristics will be described in greater detail.
In the case where a flat object is projected upon a flat imaging
plane with a usual lens, when the height of the object is small as
shown in FIG. 3a, an actual imaging plane 31 at an ideal flat
imaging 30 is curved and it takes the form of concave surface
toward the lens. However, as the height of the object increases as
shown in FIG. 3b, the actual imaging plane 31 is curved to the
ideal imaging 30 with the result that the actual imaging plane 31
as a whole approaches the ideal imaging plane 30.
On the other hand, since a lens which projects a spherical surface
object upon a flat plane is equivalent to a lens which projects a
flat plane object upon a spherical surface, there is provided, as
shown in FIG. 3c, a lens for forming an image on an actual curved
imaging surface 32 which is convex toward the lens, the actual
curved imaging surface 32 being commensurate with the spherical
phosphor screen 4a of the CRT 4. In this manner, it is possible to
correctly project an object or a picture displayed on the spherical
phosphor screen 4a of the CRT 4 upon the ideal flat imaging plane.
One example of a lens for effecting such a function is illustrated
in FIG. 4. It is seen from the figure that the first lens 21 and
the sixth lens 27 are positive lenses having a relatively strong
refractive power, the second lens 21 and the fifth lens 26 are
negative lenses, and the third lens 23 and fourth lens 25 are
positive lenses having a relatively weak refractive power. The
first lens 21 and second lens 22 are coupled, and the fifth lens 26
and sixth lens 27 are also coupled. The first, second and third
lenses are arranged to oppose the fourth, fifth and sixth lenses
along with an aperture 24 interposed therebetween. Numeral 28
designates the optical axis of the lens system.
Correcting means for compensating for reduction in illumination
intensity of a projected image at its peripheral portion will be
described hereunder.
Referring now to FIG. 5, the principle on which the present
invention is based is as follows.
Distribution of luminous flux emanating from a bright spot on the
CRT 1 is proportional to COS.sup.2 .theta., and the quantity of
light at periphery of the lens 2 decreases in proportion to
cos.sup.4 .theta.', where .theta. represents an angle between the
normal to the panel of the CRT 1 and a luminous flux toward the
lens 2 and .theta.' an angle between the optical axis of the lens 2
and the luminous flux.
In consequence, illumination intensity distribution on the imaging
plane 3 is given by,
where E represents illumination intensity, Eo illumination
intensity on the optical axis, X' distance between a projecting
point and the optical axis on the imaging plane, and A and B
proportional constants. (A >> B).
Accordingly, in order to equalize illumination intensity at a
peripheral portion of imaging plane 3 to that at center portion
thereof, it is necessary to make the brightness of a picture
displayed on the CRT 1 substantially proportional to (1 + AX.sup.2
+ BX.sup.4), where X represents a distance between a bright spot
and the optical axis on the CRT.
With reference to FIG. 6, illustrating a block diagram of one
embodiment of a CRT picture recording apparatus according to the
present invention, a signal from a deflection circuit 60 of the CRT
1 operates a brightness correcting circuit 61 whose output is
applied to a video circuit 62, so that a brightness signal
corrected in accordance with deflecting positions is added to a
video signal of the CRT 1. With this arrangement, a picture
displayed on the CRT 1 is recorded on a recording device 63 through
the lens 2.
There is shown in FIG. 7 one example of a circuit diagram of the
brightness correcting circuit 61, video circuit 62 and CRT 1 shown
in FIG. 6.
In FIG. 7, symbol HT designates an output winding of a horizontal
output transformer, K a cathode of the CRT 1, G1 the first grid of
the same, Q1 to Q5 transistors, C1 to C6 capacitors, R1 to R13
resistors, VR a variable resistor, and +B a power supply.
With this construction, across the output winding HT of the
horizontal output transformer incorporated in the deflecting
circuit 60 of FIG. 6 is created a negative pulse voltage signal 64
the period of which is identical to a horizontal deflection period.
The signal 64 is converted into a parabolic voltage signal 65, the
period of which is identical to a horizontal deflection period, by
means of a circuit including the resistors R1 to R5, capacitors C1
to C3, and transistors Q1 and Q2. That is, by integrating twice the
pulse voltage signal 64 through an integrator including two of sets
of resistor R1 and capacitor C1, and resistor R3 and capacitor C2,
the parabolic voltage signal 64 is obtained at the base of the
transistor Q2, which parabolic voltage signal is in turn applied to
the resistor R6 connected to the collector of the transistor
Q2.
On the other hand, a vertical deflection voltage signal 66 of
vertical deflection period is applied to the base of the transistor
Q4, and it is converted into a parabolic voltage signal 67 of
vertical deflection period through a circuit including the
resistors R10 to R13, capacitors C5 and C6, and transistors Q4 and
Q5. That is to say, by integrating the voltage signal 66 through an
integrator circuit including the resistor R11 and capacitor C5, the
parabolic voltage signal 67 of vertical deflection period is
created across the resistor R6 connected to the collector of the
transistor Q5.
Thus, across the resistor R6 are developed the parabolic voltage
signal 65 of horizontal deflection period and the parabolic voltage
signal 67 of vertical deflection period.
Additionally, an output voltage from the output winding HT of the
horizontal output transformer is smoothed by the diode D1 and
capacitor C4 so as to produce a negative voltage. A resultant
voltage of this negative voltage and the parabolic voltages
appearing across the resistor R6, which is obtained by the
resistors R7 and R8 and variable resistor VR, is applied to the
first grid G1 of the CRT 1. On the other hand, to the cathode K of
the CRT 1 is applied a video output from the video output
transistor Q3 of the video circuit 62 shown in FIG. 6.
A voltage to the first grid G1 is rendered negative by the resistor
R7 irrespective of values of the variable resistor VR, and a bright
spot of the CRT 1 luminesces when an output of the transistor Q3,
i.e. a voltage at the cathode K becomes lower than the voltage of
the first grid G1. The more the first grid G1 becomes negative, the
more dark the bright spot is. The smaller a negative voltage at the
first grid G1 becomes, the more bright the bright spot is. In other
words, brightness is proportional to a cathodegrid voltage.
In this manner, during horizontal and vertical deflection periods,
to the first grid G1 are applied parabolic voltage signals 65 and
67 which are low at the center of deflection and high at peripheral
portions of deflection. Thus, bright spots on the CRT 1 become more
bright from the center of the CRT toward peripheral portion
thereof. The amount of this brightness correction can be made
square proportional to a distance between the center of the CRT
panel and the peripheral portion thereof.
As described above, brightness of the bright spot image on the
imaging plane of the recording device is depicted as shown in FIGS.
8a and 8b.
More particularly, FIG. 8a shows illumination distribution of the
bright spot images on the central portion of the imaging plane, and
FIG. 8b shows illumination distribution of the bright spot images
on the peripheral portion of the same, wherein dotted curved line
80 corresponds to the illumination distribution obtained when the
brightness correction is omitted and solid curved line 81
corresponds to the illumination distribution obtained when the
brightness correction is effected in accordance with the present
invention.
As understood from the figures, peak of the illlumination
distribution at the peripheral portion obtained when no brightness
correction is effected is far smaller, as shown at dotted curved
line 80, than that of the illumination distribution 82 at the
central portion shown in FIG. 8a. As a result, for exposure level
83, the recording of the bright spot images is possible at the
central portion but impossible at the peripheral portion, and for
exposure level 84, the recording at the peripheral portion becomes
possible, while recording width for the central portion increases,
thereby impairing resolution at the central portion and preventing
its practical use.
In contrast therewith, the illumination distribution with
brightness correction, as shown at solid curved line 81, is
substantially identical with the illumination distribution 82 at
the central portion, and for exposure level 83, substantially
identical recording can be effected at both the central portion and
the peripheral portion.
While, in the foregoing, one embodiment has been described wherein
the brightness correction is effected in both the longitudinal and
transverse directions of the picture, the same effect may be
brought about when the brightness correction is effected in one
direction of the picture.
As has been described, according to the present invention, there is
provided an inexpensive CRT picture recording apparatus capable of
recording uniformly picture images at both central portion and
peripheral portion.
* * * * *